1. Field of the Invention
The present invention relates to an electromagnetic converter that carries out sound reproduction from an audio signal by using a combination of a permanent magnet and a diaphragm.
2. Description of Related Art
As conventional speakers having a structure of forming a diaphragm and a voice coil pattern into an integral piece, there have been provided, for example, a plane type sound converter as disposed in patent reference 1 and a speaker as disclosed in patent reference 2. The plane type sound converter as disposed in patent reference 1 has a diaphragm which is formed by forming a spiral voice coil pattern on a sheet made from polyimide which is a thermosetting resin, polyethylene terephthalate (PET) which is a thermoplastic resin, or a liquid crystal polymer, and bending both sides of the sheet in such a way that the bent sheet has a U-shaped cross section. A top portion of the diaphragm and a frame are joined to each other by using an edge having elasticity, and they are held in such a way that the voice coil pattern on both the side surfaces of the diaphragm is positioned in a region where a flux of magnetic induction occurs in the magnetic circuit. Therefore, even if the amplitude of the diaphragm becomes large, the lengths of up-and-down displacements of the voice coil pattern can be made to fall within the region where the flux of magnetic induction occurs.
A problem is however that because in this diaphragm only both the sides in a direction of the length of the diaphragm are bent, but both sides in a direction of the width of the diaphragm are not bent, the rigidity of the diaphragm is low and an unusual sound easily occurs due to unnecessary resonance caused by the vibration of the diaphragm. A further problem is that because a holding mechanism including the edge for fixing the diaphragm to the frame cannot be joined to anything other than the top portion of the diaphragm, the piston vibration of the diaphragm fluctuates at the time when the diaphragm has a large amplitude, and a magnetic gap failure easily occurs due to rolling.
In contrast with this, the speaker as disclosed in patent reference 2 has a dome-shaped diaphragm having a U-shaped cross section which is formed by forming a voice coil pattern on a polyimide sheet or a thermosetting resin sheet made from a glass resin and an epoxy resin, such as a prepreg laminated sheet, in advance, and then hot-forming them. While the perimeter of the diaphragm and a frame are joined to each other by using an edge, a holding plate is fixed to an inner portion of the diaphragm and is also joined to a damper disposed on the frame. Therefore, the rigidity of the diaphragm can be improved because the diaphragm is formed into a dome shape, and the holding structure for holding the diaphragm can be redundant, but partially.